1. Field of the Invention
The present invention is directed to a power conversion system which utilizes reversible energy contained in liquefied natural gas. More particularly, the present invention is directed to a combination of a fuel burning power generating plant and a large scale liquefied natural gas vaporization plant wherein reversible energy contained in the liquefied natural gas is utilized for generation of power.
2. Description of the Prior Art
Power generating plants of the prior art include a combustion chamber wherein a fuel such as natural gas, liquid hydrocarbons or coal mixed with air is burned to generate hot gaseous combustion products at high pressure. The hot gaseous combustion products are directed to drive a turbine producing rotating shaft power which is available to perform useful work such as turning an electric generator.
It has been known in the art for a long time that in order to obtain acceptable performance of the turbine, the incoming air must be compressed to a relatively high pressure prior to actual combustion of the fuel in a combustion chamber of the turbine. Accordingly, state-of-the-art power generating plants include a compressor which pressurizes the incoming air. The compressor is usually driven by the rotating shaft power of the turbine. It is well known in the art that the power input of the compressor usually consumes approximately 2/3 of the rotating shaft power produced by the turbine. Thus, the efficiency of the above described state-of-the-art power generating plants based on theoretical heat content of the fuel is necessarily limited to approximately 33% by the large power input requirement of the compressor.
Relatively recently, transoceanic transportation of liquefied natural gas has gained increasing socio-economic and technological acceptance. At the present, numerous facilities wherein liquefied natural gas is unloaded from ships and is converted to gaseous form for distribution in pipelines or for storage in a storage facility, are contemplated or are under construction. In its liquid state, the natural gas represents a substantial amount of reversible energy. It is generally recognized that approximately 300 KW (Kilowatt) of energy is required to liquefy 1.0 million standard cubic foot (MMSF) of natural gas per day. Approximately 120 KW of this energy is reversible. Therefore, in an ideal recovery system approximately 120 KW of energy may be extracted from each MMSFD of liquefied natural gas while the same is vaporized.
The presence and nature of this reversible energy in liquefied natural gas is well understood by those skilled in the engineering arts and particularly in the science of thermodynamics and does not require elaborate explanation in the present disclosure. Nevertheless, for the sake of a clear understanding and appreciation of the present invention the nature of this reversible energy is briefly explained as follows. According to basic laws of thermodynamics whenever a temperature differential exists between two bodies, useful work may be theoretically obtained in a process wherein heat is transferred from the warmer body to the colder body. The theoretical efficiency of this process is determined by the equation: ##EQU1## wherein ".eta." is a dimensionless number and T.sub.1 represents the temperature of the colder body expressed in absolute degrees, and T.sub.2 represents the temperature of the warmer body expressed in absolute degrees.
Thus, the significant temperature differential between the liquefied natural gas which is customarily transported at approximately -260.degree. F. and at atmospheric pressure, and the ambient environment represents a significant source of recoverable energy. According to presently prevailing practice in the art, liquefied natural gas is vaporized in heat exchangers which utilize large amounts of sea water as a source of heat. In this process sea water, which is cooled below its original temperature by the liquefied natural gas, is simply pumped back into the ocean thereby totally wasting the reversible energy contained in the liquefied natural gas.
In recognition of the wasteful nature of the above described method of vaporizing liquefied natural gas, several processes have been devised in the prior art to capture at least a portion of the reversible energy of the liquefied natural gas.
In one relatively simple process described in U.S. Pat. No. 3,438,216, liquefied natural gas is vaporized by exchange of heat with a large quantity of air. The air is cooled in the process to about 0.degree. F. This cold air is introduced into a compressor which is associated with and driven by a fossil fuel powered turbine. Since compression of cold air requires less power than compression of warm air, the amount of power taken by the compressor from the turbine is decreased as compared to a compressor utilizing air of ambient temperature. Accordingly, some of the reversible energy of the liquefied natural gas is utilized in this process. In other words, the efficiency of the gas turbine is increased because less power derived from the combustion of the fossil fuel is diverted for driving the compressor. Hot exhaust gases of the turbine are then utilized in a heat exchanger to bring the vaporized natural gas to a temperature of approximately 60.degree. F. The natural gas is then released for distribution in a pipeline.
In a variation of the process described in U.S. Pat. No. 3,438,216, ambient air is cooled to approximately -120.degree. F. in a heat exchanger wherein the liquefied natural gas is vaporized. Air at approximately -120.degree. is then fed into the compressor which is associated with and driven by a fossil fuel powered gas turbine.
In still another process of the prior art, which is commonly known in the art as a "Closed Brayton Cycle" or "Lotepro/Linde Cycle", a portion of the reversible energy contained in the liquefied natural gas is captured in the following manner. An inert gas such as nitrogen is utilized in a first heat exchanger to vaporize liquefied natural gas. The cooled inert gas is introduced into a compressor driven by a turbine. The compressed, cold inert gas is then led into a second heat exchanger where it is preheated by hot exhaust of the turbine. The compressed inert gas is led from the second heat exchanger into an externally fired heater wherein it is brought to a high temperature and pressure and thereafter is released to drive the turbine. From the turbine, the expanded but still hot inert gas is introduced into the second heat exchanger wherein it preheats the compressed and cold inert gas.
Additional disclosures relating to use of a cryogenic liquid such as liquid oxygen or liquefied natural gas in power conversion systems are found in U.S. Pat. Nos. 3,775,976; 3,134,228 and 3,621,656.
As is well known by those skilled in the art, the above described processes for capturing the reversible energy of liquefied natural gas have not gained wide practical acceptance. This is probably due to the following. The amount of reversible energy of the liquefied natural gas which is actually captured by these prior art processes, at least until the present, did not appear to justify the expense of providing a relatively complex power generating facility associated with a liquefied natural gas vaporizing facility and adapted for capturing this reversible energy.
The present invention is directed to a power conversion system of high efficiency which greatly increases the economic acceptability of capturing the reversible energy of liquefied natural gas in a power generating facility associated with a liquefied natural gas vaporization plant.